A display device wherein an optical conductive layer and a light modulating layer are laminated, whose optical conductive layer is exposed in accordance with image information while electric field is applied thoroughly and in whose light modulating layer images are written by controlling orientation of liquid crystals in the light modulating layer is practically used as a liquid crystal light valve or the like, as detailed processing is not necessary with it and images of high resolution can be written in it at high speed.
Conventionally, as such devices, those described in Japanese unexamined patent publication No. Hei 4-94281, Japanese unexamined patent publication No. Hei 4-130420 and Japanese unexamined patent publication No. Hei 4-218021 are known. In these publications, it is shown that laser beam is used for a light source used for optical addressing and that the purpose of these inventions are to obtain images of high resolution at low manufacturing cost (as described in Japanese unexamined patent publication Hei 4-94281) and to carry out local rewriting by controlling synchronously exposure and applying pulse of voltage of opposite polarities (as described in Japanese unexamined patent publication No. Hei 4-218021) or the like, and structures and driving methods to realize these purposes are proposed. Further, in Japanese unexamined patent publications No. Hei 3-5729 and Hei 4-53928, cases are described where amorphous silicon is used for photoconductive layer and images are written in a light modulating layer (liquid crystal layer) by scanning a laser beam. In these publications, provision of a new diode layer or a Schottky layer for the purpose of preventing applying DC voltage component, which is caused by the rectifying characteristics of Schottky structure caused by contacting the photoconductive layer and an electrode, to the light modulating layer.
On the other hand, demands for electronic devices whose displays are clear or which are easy to treat are increasing recently so that users can use devices more comfortably. Of these devices, as electronic displays are often used for operations mainly including making and displaying documents, the realization of the one capable of photosensitive displaying which does not tire human eyes, of high speed displaying, and of having high reflection contrast and memory characteristics with respect to the contents of display is desired. Display technique items which satisfy a part of these requests have been proposed, and of them, displaying using light scattering mode of liquid crystals is known.
A display of light scattering mode is made by combining liquid crystals and transparent materials whose index of refraction becomes equal to that of the liquid crystals when the orientation of the liquid crystals is perpendicular to an electrode, and it uses the characteristics of this combination wherein light is transmitted when the orientation of the liquid crystals is perpendicular to the electrode and light is scattered and not transmitted when the orientation of the liquid crystals is random because of the difference of the indexes of refraction. As this display is reflective, it does not tire human eyes, and further, as a polorizer is not necessary with this display, the amount of transmitted light is high and high reflection contrast can be obtained.
As a method for making a material used for such a display of light scattering mode, the one wherein capsulized liquid crystals are dispersed in polymer as drops of liquid crystals and the mixture is filmed is known (as described in Japanese unexamined patent publication 58-501631, which corresponds to U.S. Pat. No. 4,435,047) Here, the liquid crystals in Nematic Curvilinear Aligned Phase (NCAP) are in nematic phase showing positive dielectric anisotropy at ambient temperature, and if electric field is applied to them, the orientation of the liquid crystals is toward the electric field, and the index of refraction n.sub.0 of the liquid crystals and that of the polymer n.sub.p become equal, which makes the film transparent. If the electric field is removed, the orientation of liquid crystals becomes random, light is scattered at the borders between the liquid crystals and polymer in the film and not transmitted, and the film becomes opaque, which makes information displayed. Japanese unexamined patent publications No. Sho 63-501512 and No. Sho 63-155022 disclose the use of liquid crystals which are in smectic phase at ambient temperature, which adds to the layer memory characteristic with respect to light modulation. If these materials are used, as it is possible to store written images without a power supply by the memory characteristics of liquid crystals and to film recording and displaying medium, a screen can be treated as a hard copy.
With the structure of the above described light modulating layer wherein liquid crystals are dispersed in polymer and on which photoconductive layer is laminated, images of high contrast and high resolution can be obtained with low energy and at high speed. The medium wherein information is stored disclosed in Japanese unexamined patent publication Hei 3-155525, which is an example of conventional arts, comprises a light modulating layer of high resin having liquid crystals dispersed therein and a photoconductive layer comprising amorphous silicon, and with it, optical images are focused on the photoconductive layer and intensity of electric field which depends on the exposure pattern to each portion of the photoconductive layer is provided to the light modulating layer. As the source of light used for addressing to this light modulating layer on which the photoconductive layer is laminated, may be used a combination of laser light source which emits in accordance with image signals in time series and some optical scanning devices, in addition to an emitting device made by applying CRT to it or the like. With a combination of laser light source which emits in accordance with image signals in time series and some optical scanning devices, it is possible to realize easy outputting of electronic image information, and more general usage can be expected.
With such devices, however, if images are written by applying AC electric field while scanning a beam, non-uniformity of density occurs in written images, because of the waveform of applied electric field, and irregularity of timing when exposing and applying the electric field are carried out.
The cause of non-uniformity of density is now described, referring to an example. Conventionally, in many cases, in order to control the transmissivity of a light modulating layer comprising liquid crystals dispersed in high polymer, resin AC electric filed is applied for the purpose of preventing deterioration, which is caused by DC component, or the like of the light modulating layer. When using a film of high polymer resin having liquid crystals dispersed therein, if a voltage is applied, the liquid crystals in the high polymer resin are oriented and the difference of indexes of refraction at the borders between the liquid crystals and the high polymer resin become small enough to be disregarded, which makes the film transparent. If electric field is not applied, high polymer resin and liquid crystals are in optically anisolotropic state and becomes light-scattered. Here, if the intensity of electric field is different portion by portion, the orientation of liquid crystals is also different portion by portion in accordance with the state of the portions, which causes non-uniformity of density of images. Further, if the polarity of applied voltage is the same continuously, DC component occurs, which causes resolution or the like of the material of the liquid crystals and deterioration of characteristics of the liquid crystals. When scanning a beam on the photoconductive layer by means of laser beam or the like is repeated while applying AC electric field to the light modulating layer on which the photoconductive layer is laminated, the voltage applied to a pixel (hereinafter in this specification, pixel corresponds to the portion to be exposed) does not comprise waveforms having continuously same effective values but of short waveforms corresponding to short exposure time.
Therefore, as a conventional example whose schematic structure is shown in FIG. 2, when a beam is scanned by exposing means 4 to a medium wherein information is stored and which comprises light modulating layer 1 including of high polymer having liquid crystals dispersed therein and optical conductive layer 2, if writing of images is carried out without controlling the polarity of voltage applied from AC power supply 31 or waveform, effective values or polarity of pulse of voltage repeatedly applied to the same pixels may be different each time. No attempt are tried in the above described conventional arts to overcome the non-uniformity of density caused by the combination of scanning exposure and applying of AC electric field and the deterioration of light modulating layer caused by applying DC component.